The invention relates to a method for determining an imbalance characteristic of a hairspring-balance oscillator of a horology movement. It also relates to a method for regulation of a hairspring-balance oscillator, comprising implementation of a method for determination of this type. It also relates to a balance or a hairspring-balance oscillator obtained by means of implementation of a method for regulation of this type, and a movement or a horology piece comprising a balance or a hairspring-balance oscillator of this type.
Balancing of the balance is one of the most important steps of production of a hairspring-balance oscillator which is designed to equip a horology movement. In fact, in an ideal situation, the center of gravity of the balance must be on its axis of rotation, under penalty of inducing defects which quickly become detrimental for the chronometry of the movement. The conventional machining techniques are in general not accurate enough to guarantee good balance of the balance, and this balance is further modified by rendering the balance integral with the other components which form the hairspring-balance (driving of the staff, plate, collet, hairspring). An imbalance measurement and subsequent correction are in general undertaken on the balance provided only with its staff and the plate, before pairing with the hairspring and assembly in movement.
This balancing of the balance alone makes it possible to obtain good chronometric performance, but scope for improvement remains in view of the residual imbalance which persists and/or is generated by the driving in of the hairspring. Solutions for balancing of the assembled hairspring-balance oscillator in motion exist (“dynamic balancing”), but these are unsatisfactory, since they can give rise to deterioration of the chronometry instead of resulting in the improvement required.
The static imbalance of the balance characterizes the off-centering of the center of gravity of the balance relative to the axis of rotation. This imbalance is the product of the mass of the balance times the distance between its center of gravity and the axis of rotation. In the case of horology balances, the imbalance is typically measured in μg·cm or nN·m. With terrestrial gravity, 1 μg·cm corresponds to approximately 0.1 nN·m.
It is found that:                The effect of the imbalance on the rate is proportional to the imbalance itself.        The effect of the imbalance is inversely proportional to the inertia of the balance. It will therefore be all the greater, the lower the level of inertia.        The effect of the imbalance is highly dependent on the amplitude of oscillation of the balance. It is even cancelled out completely for an amplitude close to 220°.        The effect of the imbalance varies as the sine of the azimuth angle between the axis of the balance (in general the direction at right-angles to the plane of the movement) and the vertical.        The effect of the imbalance varies with the angle between the direction of the imbalance and the vertical. For example, when the axis of the balance is horizontal, there are two opposite positions where the imbalance is cancelled out, and two positions perpendicular to these two first positions where it is maximum, but these positions are not generally the four normalized vertical positions of the watch.        
Usually, the imbalance of the balance is measured and adjusted before assembly with the hairspring. The measurement can be performed by rotating the balance around its staff placed horizontally between two bearings, and by measuring the oscillation and/or the reaction forces of the support by means of piezoelectric sensors. The imbalance value is obtained by calibration of the signal. A balancing operation is then carried out which consists of removing material from the felloe of the balance in a targeted manner.
Another possibility consists of carrying out “dynamic balancing” which consists of minimizing the rate differences between positions by modifying the balancing of the balance on the basis of measurement in motion at a given amplitude. This method is unreliable: the effect of the imbalance is not necessarily preponderant in comparison with other sources of amplitude differences for which the measurement is performed. By using the balancing to correct the sum of these effects, it is perfectly possible to worsen considerably the imbalance of the balance, which will disrupt the chronometric performance, in particular at low amplitudes. An approach of this type should therefore be avoided, and is strongly advised against in the literature.
In the article “La mise d'équilibre des balanciers” (“Balancing of the balances”), Proceedings of the Swiss Chronometry Congress 1966, p. 324, J.-J. Augsburger defines balancing defects, their effects on the rate of the movement, and the means for measuring them, as well as the balancing means available at the time. Theoretical development indicates that the effect of the imbalance is cancelled out at an amplitude of 220°, and that the effect on the rate is directly proportional to the imbalance, and is all the more noticeable, the lower the level of inertia of the balance. Careful balancing by means of milling makes it possible to bring the imbalance of a balance alone down to a mean value of 1.5 μg·cm.
In the article “L'équipement pour l'équilibrage dynamique du système oscillant balancier-spiral REGLOWITCH-M” (“The M. REGLOWITCH equipment for dynamic balancing of the hairspring-balance oscillator system”), Proceedings of the 6th European Chronometry Congress 1996, p. 153, Furer et al. describe a dynamic balancing apparatus: the rate and the amplitude of a movement are measured in the different horological positions, for a single state of winding of the barrel, and therefore at a single amplitude value situated either between 150° and 180°, or above 260°. This therefore involves conventional dynamic balancing with a measurement carried out at a single amplitude, meaning that the effect measured can very well be derived from a source other than the imbalance, and that the correction carried out on this basis has as much chance of worsening the imbalance as it does of improving it. Furthermore, the term “dynamic balancing” seems to be inappropriate, since the method described aims to adjust the difference between positions at a given amplitude, and not to balance the hairspring-balance.
The document “Traité de construction horlogère” (“Horology Construction Treatise”), Presses Polytechniques et Universitaires Romandes, Lausanne 2011, pp. 190-200, by M. Vermot et al., devotes a chapter to the balance defect of the balance alone and its consequences. The different measurement methods are reviewed. The method of “rate to positions”, which corresponds to the dynamic balancing referred to in the article “L'équipement pour l'équilibrage dynamique du système oscillant balancier-spiral REGLOWITCH-M” is mentioned: a low amplitude is recommended for the measurement in order to maximise the effects. However, it is clearly stated that this method “lacks precision because of all the hypotheses formulated for its application”, and that “in practice, it is not possible to detect imbalances which are sufficiently great in order for the effects on the rate not to be concealed in other rate variations [ . . . ]”.
U.S. Pat. No. 3,225,586 proposes a method for simultaneous regulation of the rate and “dynamic balancing” by means of four screws placed on the felloe of the balance, based on a measurement of the rate in four vertical positions. It is noted that a tool of the slide rule type makes it possible to convert the result of the measurement directly into the number of turns to be applied to each screw. The correction procedure is very specific to the measurement apparatus used (“Watchmaster”, U.S. Pat. No. 2,113,825) and cannot be adapted to more recent measurement means.
Patent application WO2012007460 is a recent example of a device for measurement and correction of the balance defect of a balance. This application describes a method for balancing the hairspring-balance assembly, in particular when the balance is fitted in a watch movement. The balancing is carried out by addition and/or removal and/or displacement of material, in particular by means of the laser machining type. Advantageously, it is recommended to carry out the measurement and/or correction of the balance at a fixed amplitude with a value of 137° or 316.5°: according to the inventors, these two amplitude values make it possible to avoid imbalance caused by the material added or removed, i.e. the centre of the mass of the material removed or added is situated at the centre of the hairspring-balance assembly. However, no details are given concerning the manner of measuring the balance defect of the hairspring-balance.